Actually, it's not
The "engine exhaust watermix elbow" is just that - the engine exhaust watermix elbow. It's connected to the riser via pipe threads. These are available from a Universal parts distributor for around $120. You might not need to replace the exhaust piping which comes off the flange, but when my elbow broke I replaced everything, from the exhaust flange at the manifold through the "hump hose" to the waterlift muffler, just so it would stay fixed for another 20 years. I used schedule 80 pipe black iron pipe (I think) which the plumbing supply house cut and threaded for me at the lengths I specified, copied from the old pieces. It takes a big vise and wrench to screw this stuff together correctly, so have someone do it for you if you don't have the setup. A plumber will be cheaper than a boatyard, hard as that may be to believe. The supply house might do it for you, which will be the cheapest. It must be wrapped properly to heat-insulate it, so it doesn't set the boat on fire or burn you. Catalina Direct has the hump hoses, which have a vibration absorbing bulge in them, unlike the straight exhaust hose originally fitted by Catalina. This job is worth probably $500 in parts and labor if the yard does it, maybe more.For general engine parts, MarineDieselDirect. To save money, find a Kubota engine dealer and determine what model Kubota yours is. An M25 is Kubota D850.